This message is quoted from "zend certification study guide" :">"By default, SPL uses its own autoloader, called spl_autoload(); this built-infunction checks all include paths for ﬁlenames that match the name of the classthat needs loading in lowercase letters, followed by .inc, .php, or the exten-sions speciﬁed using a comma-separated string as the only parameter to a call tospl_autoload_extensions()"

Regard to function spl_autoload_extensions(), it allow you to define extension of filename.